The present invention relates to an actuating suspension assembly in a disc drive, such as a hard drive using a magnetic storage medium. More particularly, the present invention relates to a disc drive microactuator such as a high resolution positioning mechanism using two-stage actuation for achieving and maintaining a low transducer portion of a slider with respect to circumferential data tracks of a rotatable disc.
As the areal density of concentric data tracks on magnetic discs continues to increase (that is, the size of data tracks and radial spacing between data tracks decrease), more precise head positioning is required. Head positioning in a disc drive includes two distinct but related aspects: tracking control (i.e., radial positioning of the head) and fly-height control (i.e., head-media spacing). Although both aspects are important considerations in disc drives in the future, the present invention mainly relates to fly-height control.
Compared to the tracking control, controlling of the height at which the head slider is floating (i.e., fly-height control) is a separate but related problem. Fly-height control itself also has two distinct but related aspects: 1) achieving a desired low fly-height during disc rotation and 2) keeping the fly-height as close as possible to a constant during disc rotation. The first aspect relates to the capability to achieve a sustainable average low fly-height while the second aspect relates to the stability of the fly-height during disc rotation regardless of the average fly-height. These two aspects are further explained as follows.
Increasingly higher areal density of data tracks in the modem disc drives requires that, in addition to having direct impact on radial positioning resolution, the fly-height be decreased in order to obtain higher signal resolution. That is, there is a pressing need for the air-bearing surface of a slider to fly as close to the media as possible, without touching the media to produce better resolution of data on the media, because read/write signal amplitude is dependent on the distance between the magnetic medium and the read/write head, and close spacing drastically improves transducer performance without having to improve sensitivity of the transducer.
Fly-height is determined by the dynamics of air bearing in modern disc drives which use a head suspension to support the magnetic read/write heads in close proximity to the rotating magnetic discs. The well known and widely used Watrous-type suspensions such as that disclosed in the U.S. Pat. No. 5,198,945 to Blaeser et al. include a load beam having a mounting region or base plate at a proximal end, a flexure on a distal end, a relatively rigid region adjacent to the flexure and a spring region between the base plate and rigid region. An air-bearing slider which includes the magnetic head is mounted to the flexure. During operation of the disc drive, typically the slider does not touch the rotating disc. Rather, the force of the air rotating with the disc causes the slider to fly at a microscopic distance known as the fly-height above the disc. A gram load force applied on the suspension urges the slider toward the disc to counteract the slider-generated air-bearing forces and maintain the proper fly height. During the rotation of the disc, the actual fly-height is determined by the air bearing design and the amount of gram load applied.
The schemes used in conventional hard drives to lower the fly-height primarily address air bearing designs. However, this approach is reaching the limit of its ability to meet the ever decreasing fly-height requirement and the accompanying need to control disturbances present in the hard drive environment.
Given an air bearing design, the average fly-height can potentially be adjusted by varying the amount of gram load applied on the suspension. In conventional hard drives, the gram load is established during the manufacturing of the hard drives. That is, each hard drive as a final product has a predetermined pre-load on the suspension load beam. Techniques for adjusting suspension gram loads during manufacturing are well known and disclosed. For example, the Schones et al. U.S. Pat. No. 5,297,413 discloses a machine capable of quickly adjusting loads on head gimbal assemblies and/or suspension assemblies so as to bring the loads within a desired specification window defined by upper and lower window specifications. The load adjusting machine includes a load cell, a mechanical bending mechanism and a heat source, all of which are coupled to and controlled by a computer. During the load adjusting process, the machine bends the suspension assembly load beams to increase and decrease loads. The process is complete when the load is within the desired specification. The bending of the suspension load beams is permanently retained. Following the manufacture and gram load adjustment of the suspensions, the sliders are bonded to the flexures, typically in a manual operation, to form head suspension assemblies. The head suspension assemblies are in turn mounted to actuator arms extending from a rotating actuator shaft to form a head stack assembly. The head stack assembly is then mounted with respect to a stack of magnetic discs, with the suspensions extending between the discs.
Given the air bearing mechanism used and the gram load applied, the average fly height of a disc drive is largely determined. Since the gram load in a conventional hard drive are is mechanically predetermined during manufacturing, the average fly-height is also mechanically predetermined.
Compared to meeting the demand for ever decreasing average fly height, maintaining a constant fly height during the operation of a disc drive is a related but different problem. As the fly-height of the head decreases, fluctuation, vibration, roughness of the disc surface and thermal effects start to play an increasingly important role, creating a more stringent requirement for fly-height stability.
In addition to random fluctuations such as that caused by mechanical or thermal noise, it is known that the air flow, which causes the slider to float, increases as the head is moved from the inner to the outer circumference of the disc. This also causes the fly height to change. In this disclosure, the phrase xe2x80x9cenvironmental fluctuationxe2x80x9d is used to include every possible type of fly-height fluctuations caused by environmental factors other than a change of gram load on the suspension load beam.
A stabilizing device is thus required in order to keep the fly-height constant as the system experiences environmental fluctuations such as mechanical and thermal noise, or the radial positioning the head slider above the disc surface changes.
Various methods existing in the prior art for controlling transducer head fly-height. For example, it is known to address the head-media spacing loss due to thermal expansion of the transducer by optimizing the thermal mechanical structure and properties of the transducer. Such a method is in essence a passive countermeasure and fails to actively adjust the pole tip position of the transducer to consistently minimize its impact on head-media spacing.
Several patents discuss the use of piezoelectric material in a slider, to adjust the position of a transducer mounted to the slider. For example, U.S. Pat. No. 5,021,906 (Chang et al.) discloses a programmable air bearing slider with a deformable piezoelectric central region between leading edge and trailing edge regions. The deformable region is controlled electrically to change the angle between the leading and trailing regions, thus to change the position of a transducer mounted to the trailing region.
U.S. Pat. No. 4,853,810 (Pohl et al.) discloses a magnetic transducing head including a body and a piezoelectric layer adjacent the body. The piezoelectric layer is operable to control the head/disc gap, based on sensing a tunnel current across the gap between the recording surface and a tunnel electrode on the slider.
U.S. Pat. No. 5,991,113 (Meyers et al.) discloses a transducer movable toward and away from the air bearing surface responsive to changes in the slider operating temperature. The transducer movement is either due to a difference in thermal expansion coefficients between a transducing region of the slider incorporating the transducer and the remainder of the slider body, or by virtue of a strip of thermally expansive material incorporated into the slider near the transducer to contribute to the displacement by its own expansion.
It has also been suggested in the prior art publications that a piezoelectric microactuator may be used to actively control fly-height. See C. E. Yeack-Scranton, IEEE Trans Magn, MAG-22 (1986), p2763 and C. E. Yeack-Scranton, et al., IEEE Trans Magn., MAG-26 (1990), p2478. The suggested schemes are based on piezoelectric strain actuators which have drawbacks including processing difficulties, compatibility problems, structure complexity, high-cost of production, low-frequency bandwidth, small fly-height or small weight control range and large operation voltages.
U.S. Pat. No. 4,605,977 (Matthews) discloses a flexible beam affixed to the slider providing a cantilever structure. A magnetic head is mounted on the free end of the cantilever. A pair of oppositely polarized piezoelectric crystals are mounted on the cantilever assembly. When energized by an electrical driving source, the cantilever beam is flexed upwards or downwards thereby changing the distance of the magnetic head from the disc.
U.S. Pat. No. 5,719,720 (Lee) describes use of piezoelectric strain effect to avoid or reduce the contact between the transducer head and surface of the disc drive. A head suspension mechanism having a unimorph piezoelectric layer is attached to a bottom surface portion of a resilient portion of a load beam, and a second layer of piezoelectric material is attached to a top surface portion of the resilient portion of the load beam, wherein a control signal induces a piezoelectric strain effect in the first layer of piezoelectric material to cause the load beam to raise the head slider from the surface of the disc in a start mode before the disc begins to rotate, and the second layer of piezoelectric material senses the strain and generates the corresponding signal to start rotation of the disc. The piezoelectric strain effect is also used to cause the load beam to keep the slider from contacting the surface of the disc until the disc comes to a complete stop.
U.S. Pat. No. 6,166,874 (Kim) further discloses an application of a microactuating scheme based on piezoelectric strain effect for actively adjusting the fly-height during the operation of the disc drive to compensate for displacements of the flying head caused by a minute impact or vibration.
Similar to the earlier proposals of using a piezoelectric microactuator to control fly-height (C. E. Yeack-Scranton, IEEE Trans Magn., MAG-22, and C. E. Yeack-Scranton, IEEE Trans Magn., MAG-26), the microactuating schemes used in the above two patents (U.S. Pat. No. 5,719,720 and U.S. Pat. No. 6,166,874) are based on piezoelectric strain actuators and thus have the same drawbacks.
Most importantly, microactuation found in the prior art usually does either or both of the following: 1) stabilizing a given average fly-height which is predetermined by the air bearing design and the gram load (with the gram load mechanically and permanently predetermined during manufacturing); and 2) temporarily lifting up a transducer head that is resting on the disc surface prior to the rotation of the disc in order to protect the transducer head and the disc surface. There is no provision in the prior art of a mechanism to achieve a desired average fly height using an electric control signal during the operation of the disc. More specifically, there is no provision in the prior art of a method adjusting the gram load on the suspension during the operation of the disc to achieve a desired average fly-height. There is further no provision in the prior art of a mechanism both achieving a desired average fly-height and stabilizing thereof during operation by preventing fluctuations. Furthermore, the piezoelectric microactuation mechanisms used in the prior art to stabilize fly-height tend to have drawbacks including processing difficulties, compatibility, structure complexity, high production costs, low-frequency bandwidth, small control range of fly-height or load weight, requirement of large operation voltages, and lack of an integrated microactuating scheme for both fly-height control radial tracking control.
The present invention discloses a novel application of a microactuating mechanism in a suspension assembly to achieve a desired fly-height of the slider, especially a very low fly height, by using a microactuator to apply a desired amount of alteration gram load on a suspension load beam. The suspension load beam has a front end connecting to a slider assembly carrying a transducer head and has a rear end. The microactuator is placed on the suspension load beam for bending the suspension load beam at the front end during a sustained period of data read/write time and thus achieving a desired average fly-height which is different from an unaltered average fly-height which would have been achieved without the microactuator.
In one embodiment, the desired low fly-height is lower than the unaltered fly-height. The invention thus provides an effective way of achieving very low fly-height in a disc drive.
Bimorph piezoelectric microactuators are preferred and placed on the rear end of the load beam to bend the load beam. In one aspect of the invention, microactuation further has a sensor and a feedback circuit for dynamically adjusting the fly-height in the disc drive such that the fly-height remains constant at the desired fly-height. In another aspect of the invention, the microactuation further has the capability of laterally bending the suspension load beam thus laterally fine positioning the transducer head in addition to controlling the fly-height.